The appearance of objects that are illuminated by a light source depends both on the properties of the object and the spectral content of the light source. The image formed when a picture is taken of a scene that is illuminated by a light source depends on both the spectral content of the light source and the reflectivity of the objects in the scene as a function of wavelength. For example, an image taken by a digital camera is typically divided into a number of pixels, each pixel representing the light received from a corresponding point in the image. The light reaching a pixel is a complex spectrum that can be represented as a set of intensity values for each of a large number of wavelengths. In the simplest case, the intensity value at any given wavelength is related to the amount of light reaching the corresponding point in the scene from the light source at that wavelength and the fraction of the light at that wavelength that is reflected by the corresponding point in the scene. The spectrum reaching each pixel is typically reduced to three color intensity values that, if input to the human eye, would cause a human observer to perceive the same color for that point that the observer would have perceived if the original spectrum at that point was directed into the observer's eye.
Similarly, consider a transparency that is illuminated from behind by a light source. In the simplest case, the light reaching the observer from each point on the transparency is related to the spectrum emitted by the light source and the absorption of light by the transparency as a function of wavelength at that point. Hence, the appearance of the transparency also depends both on the transparency and the spectral content of the light source.
To some extent, the human eye adapts to different illumination sources so that the perceived colors in a scene illuminated with light from two different light sources appear to be the same even though the light sources have different spectral content. However, cameras do not have this ability. Hence, algorithms that are designed to compensate for differences in the spectral content of the light sources are often included in modern digital cameras. Given a picture of a scene and a knowledge of the spectral content of the illumination source, the algorithm generates a new picture that ideally represents the picture of the scene that would have been produced if a standard light source had been utilized as the illumination source.
One class of light sources that are used to illuminate objects emit “white” light. These light sources emit light that a viewer perceives as approximating the spectrum emitted by a black body that is heated to a high temperature. Since the spectrum changes as a function of temperature, the output spectrum is typically specified by the temperature of the black body that emits a spectrum that approximates the spectrum of the white light source. While not all light sources that are perceived by a human observer as being a white light source of a particular color temperature have identical spectrums, the approximation is often useful.
For example, consider a picture that was taken with a particular white light source. In some cases, the user wishes to know how the image would have appeared if it had been taken by a “standard” white light source that is different than the one utilized. This is often referred to as “white balancing”. Cameras with white balance settings for illumination by sun light, fluorescent lights, and incandescent lights of various color temperatures are known. Given a knowledge of which light source was used, the camera can rebalance the color of the image to provide the image that would have been seen if the standard light source had been utilized.
In such cameras, the user must decide which of the settings correspond most closely to the lighting conditions present at the time the image was taken. If the user cannot decide on the correct setting, or if none of the settings are a good approximation to the light source used, effective color compensation cannot be performed in the camera. Furthermore, compensation by a separate image processor is also not possible, since the separate image processor lacks any measurement of the actual spectrum used to form the image in the first place. If the camera had a sensor that measured the color temperature of the light source, the camera could automatically generate the data needed to perform the color balancing operation.
Many digital cameras provide an automatic white balancing setting that utilizes the spectrum content of the image itself to determine the spectral content of the illumination source. The algorithms utilized assume that the spectral content of the image as a whole is representative of the spectral content of the light source. Such systems perform poorly if the image is biased toward a particular color. To provide better color balancing, the camera needs to have a measure of the spectral content of the illumination source independent of the particular scene being imaged.
In addition, many light sources have adjustments that allow the effective color temperature of the light source to be adjusted. For example, the current through the filament of an incandescent lamp can be increased or decreased to alter the color temperature of the light from the lamp. Similarly, solid-state light sources that utilize LEDs of different colors to generate white light can be adjusted by altering the power delivered to the various colored LEDs.
In many cases, the output of a light source drifts over time, and a servo loop is used to correct for the drift by altering the parameters that control the color temperature of the light source. This type of servo also relies on a light sensor that detects the color temperature of the light source.
The color temperature and intensity of the optimum light source for illuminating a backlit displace such as an LCD display depends on the color temperature and intensity of the ambient light in the room in which the display is located. It has been found that the color temperature and intensity can be altered to provide a more comfortable viewing experience for a human observer. Hence, a sensor that measures the color temperature and intensity of the room lighting is needed in systems that alter the display color temperature in an attempt to realize a more optimum illumination.
In principle, a color temperature sensor could be constructed from a number of photodetectors that measure the output of the light source in a number of different wavelength bands. Sensors based on photodiodes that are covered with bandpass filters that determine the wavelength band viewed by each photodiode are known to the art.
However, a color temperature sensor for use in consumer products is constrained by the cost of the sensor. Typically, the consumer products of interest depend on custom integrated circuits that are fabricated in CMOS. Hence, to reduce the cost of the sensor, the sensor should be capable of being fabricated in CMOS as part of the custom IC that implements the other functions inherent in the product. The type of multi-photodiode sensor discussed above is not easily implemented in this technology, because each photo-diode must be covered with a layer of material that provides the bandpass filter function for that photodiode, and hence, a number of additional masking and deposition steps are required after the integrated circuit having the photodiodes is finished.
In addition, at least 3 photodiodes are used with such sensors together with computational hardware that corrects the output of the photodiodes to provide the intensities of the light source at three colors that can be used to compute a location in a conventional color space, which, in turn, is used to compute the color temperature of the light source.
As noted above, in some applications the intensity of the ambient light must also be measured. The intensity value that is needed is the value that matches the perceived intensity as viewed by a human observer. Intensity values based on a single photodiode do not provide intensity measurements that agree with the intensity perceived by a human observer when the spectral content of the light source varies. For example, consider the case of a fluorescent light and an incandescent light that are perceived by a human observer as being of the same intensity. The intensity values generated by a single photodiode for these two sources differ significantly. Hence, a sensor that is to measure both color temperature and light intensity as perceived by a human observer requires additional photodiodes.